Traditionally, considerations of the roles played by the medial gastrocnemius (MG) muscle during stepping have been limited to extension of the ankle joint, either actively during the late swing phase or as an elastic body during the stance phase. The main objective of the proposed study is to consider the ankle extensor function of MG in more detail then reported previously and to consider possible roles other than simple talocrural extension. Normal locomotor function is examined using a combination of biplanar high-speed cinematography, force platform analysis, and multichannel electromyography. Limb movements (joint angle changes and body segment motions) and activity patterns in MG and all of its possible synergists are studied before and after partial and total denervations of MG. Analysis of this kinetic and kinematic data permits an accurate correlation of biomechanical events of a normal step cycle and the roles of MG as elaborated by the central nervous system. Specifically, the results of preliminary pilot studies indicate that MG functions (1) to produce acceleration of the talocrural joint during the swing phase of stepping so that the foot is moved with a large average velocity, (2) to decelerate the knee joint just prior to foot placement, and (4) to extend the foot and provide forward thrust via an elastic stretch-shorten mechanism during the stance phase. The applicability of studies of both peripheral and central aspects of the locomotor system in cats to that of man has a long and well-established history. It will contribute information as to the roles played by individual synergistic muscles during stepping, provide valuable peripheral correlates to the central program for stepping, and provide information on normal muscle function which can be used by specialists in rehabilitation medicine.